1. Field of the Invention
The invention relates to a liquid crystal display (LCD) that has a high aperture ratio (HAR), and more particularly, to a pixel electrode structure for the LCD with HAR.
2. Description of the Related Art
As resolutions of liquid crystal display (LCD) increase, aperture ratio of the LCD becomes more and more insufficient. Recently, color filter on array (COA) technology, has been developed to provide a LCD of high aperture ratio. In the HAR process, the color filter process is integrated with the TFT array process on the same glass substrate, thus the aperture ratio of the TFT-LCD device is increased to effectively improve brightness of the panel, and the step of attachment/alignment between a color filter substrate and a TFT array substrate is omitted to improve yield and decrease process costs. Moreover, in the HAR process, an overlapping portion is formed between a transparent pixel electrode and a data line to decrease the required area of a black matrix (BM) layer, resulting in a higher aperture ratio in the TFT-LCD device.
FIG. 1 is a top view showing an electrode structure in a pixel of a conventional TFT-LCD device formed using a HAR process. FIG. 2 is a sectional diagram along line I-I of FIG. 1 showing the rotating orientations of liquid crystal molecules in the conventional TFT-LCD device using a HAR process.
The conventional TFT-LCD device comprises a plurality of traversing gate lines 12 and data lines 14 extending lengthwise to define a plurality of pixels 10 in a matrix, each pixel 10 comprising a pixel electrode 16 and a TFT. Using one data line 14 as the boundary, a first pixel 10A is covered by a first pixel electrode 16A, and a second pixel 10B is covered by a second pixel electrode 16B. Also, the periphery of the first pixel electrode 16A overlaps one side portion of the data line 14, the periphery of the second pixel electrode 16A overlaps another side portion of the data line 14, and a predetermined distance is kept to space the first pixel electrode 16A from the second pixel electrode 16B over the data line 14. Further, an arrow P indicates a light-polarization planar direction on a polarizer, a light-entry direction.
During a HAR process on a TFT array glass substrate 17, a first metal layer is patterned as the gate lines 12, and then an insulating layer 15 is deposited to cover the gate lines 12 and the glass substrate 17. Next, processes corresponding to TFT are performed on a predetermined area of the gate line 12, and a second metal layer is patterned as the data lines 14. Next, a transparent conductive layer is patterned as the first pixel electrode 16A and the second pixel electrode 16B.
In a case using TN-type (twisted nematic type) LCD, when an extra voltage exceeds a critical value, the liquid crystal molecules 18 originally parallel to the alignment film are rotated to become perpendicular to the alignment layer in accordance with the magnitude of the lengthwise electric field. However, a transverse electric field is generated between the periphery of the first pixel electrode 16A and the periphery of the second pixel electrode 16B, thus the inclined directions of the liquid crystal molecules 18I and 18II near the periphery of the pixel electrodes 16A and 16B are influenced by the lengthwise and transverse electric field. Also, when an included angle between the arrow P (a light-polarization planar direction on a polarizer) and the long-axis direction of the liquid crystal molecule 18I is 45°, an ellipsoidal polarized light caused by birefringence effect may pass an analyzer perpendicular to the polarizer to result in light leakage. With regard to the rotating orientation of the liquid crystal molecules 18II over the sidewall of the data line 14, however, a Mura phenomenon occurs, manifesting as non-uniform color difference, to form a light leakage area L.
In another attempt to solve the above-described problems, the width of the data line 14 is increased to shield the light leakage area L, but this decreases the aperture ratio of the TFT-LCD device.